1. Field of the Invention
The present invention relates to a coated optical fiber. More particularly, the present invention relates to a coated optical fiber where a secondary layer of the coated optical fiber contains pigment and is colored by the pigment, and an optical fiber unit which is constructed by a plurality of the coated optical fibers using photo setting resin.
2. Description of the Art
FIG. 1 is a cross sectional view showing a structure of a typical coated optical fiber to which the present invention is applicable.
Generally, a coated optical fiber 11 is made up of a secondary layer 3 and an optical fiber strand 10 including an optical fiber 1 made of SiO.sub.2 as a main component and a primary layer 2 made of material such as silicone resin or ultraviolet curable resin. The primary layer 2 coats the optical fiber 1, and the secondary layer 3 made of thermoplastic resin, e.g., nylon, coats the optical fiber strand 10. The resin of covering layers 2 and 3 is originally transparent, however where a plurality of coated optical fibers are gathered, as will be described below, the coated optical fiber 11 is frequently colored by mixing pigment into the secondary layer 3, in order to distinguish the coated optical fibers from one another.
Generally, the coated optical fiber thus constructed is dimensionally designed such that the diameter of optical fiber 1 is approximately 125 .mu.m, the diameter of a strand as formed by covering the fiber with primary layer 2 is approximately 400 .mu.m and the diameter of a coated optical fiber formed by covering the strand with secondary layer 3 is approximately 600 to 900 .mu.m.
The coated optical fibers are usually used in such a way that a plurality of coated optical fibers are collectively arranged into an optical fiber unit or an optical fiber cable.
FIG. 2(a) is a cross sectional view showing a typical structure of an optical fiber unit using coated optical fibers. As shown, in an optical fiber unit 20, a tensile strength member 21 is disposed at the center. A plurality of coated optical fibers 11 are disposed around the member 21. Ultraviolet curable resin 22 is applied to an arrangement of the tensile strength member 21 and the coated optical fibers 11, and is cured into a single solid structure.
In manufacturing the optical fiber unit 20, coated optical fibers 11 are twisted and disposed around the tensile strength member 21. The ultraviolet curable resin 22 is then applied to an arrangement of the fibers and the member 21, forming a single solid structure. Finally, the ultraviolet curable resin layer 22 is cured by irradiating it with ultraviolet rays.
In the above described optical fiber unit, a coated optical fiber is covered with a plurality of covering layers, to form an optical fiber. A plurality of coated optical fibers are gathered, twisted, disposed around a tensile strength member, and covered again with ultraviolet curable resin. Even with such a solid structure, when the fiber unit is bent, transmission loss thereof is often remarkably increased.
Such a tendency of the fiber unit limits the practical bendable range of the fiber unit. This is one of the problems urgently needed to be solved in this technical field.